Plasma Diffusion across a Magnetic Field Due to Thermal Vortices

A convergent cross-field diffusion coefficient due to flute-type thermal fluctuations (vortices) is derived, taking into account the nonlinear, orbit-diffusion effect on the dynamic screening of the interaction and the fluctuation spectrum. Because of the finite-gyroradius reduction of the correlation, there is a region where the diffusion coefficient depends on $B$ only logarithmically.